Such illumination systems are known per se. They are used, inter alia, as luminaire for general lighting purposes, for example, for office lighting or for shop lighting, for example, shop window lighting or lighting of (transparent or semi-transparent) plates of glass or of (transparent) plates of glass or of (transparent) synthetic resin on which items, for example jewelry, are displayed. An alternative application is the use of such illumination systems for illuminating advertising boards, billboards as a display device.
The known illumination systems are also used as light sources in backlighting systems in (picture) display devices, for example, for TV sets and monitors. Such illumination systems are particularly suitable for use as backlighting systems for non-emissive displays such as liquid crystal display devices, also denoted LCD panels, which are used in (portable) computers or (portable) telephones.
Said display devices usually comprise a substrate provided with a regular pattern of pixels which are each controlled by at least one electrode. The display device utilizes a control circuit for achieving a picture or a data graphical display in a relevant field of a (picture) screen of the (picture) display device. The light originating from the backlighting system in an LCD device is modulated by means of a switch or modulator in which various types of liquid crystal effects may be used. In addition, the display may be based on electrophoretic or electromechanical effects.
Such an illumination system for illuminating an image display device is known from the US patent application US 2006/0262310. The illumination system comprises two light emitting diodes providing light to an optical cavity. The diodes may be colored or white. The optical cavity is lined with a diffuse reflector. The diffuse reflector both increases reflectance and mixes the discrete light colors adequately to form a white light source for illuminating a liquid crystal display device. A disadvantage of the known illumination system is that the uniformity over the light-emitting window is relatively poor.